Intracavity doubled Nd:YAG lasers were proposed as sources of green light more than 20 years ago, and many such devices have been built and analyzed in the ensuing years. Typical devices consisted of a Nd:YAG rod, a Brewster polarizer and a Type-I phase-matched crystal, such as Ba.sub.2 NaNb.sub.5 O.sub.15 or LiIO.sub.3. Several examples of this type of device are shown in the book by Koechner, Solid State Laser Engineering, Springer-Verlag, 2nd edition, 1988. In general, it was observed that these devices were much less stable with the non-linear crystal in the cavity than they were without it. Several tentative explanations involving modebeating or thermal effects were suggested, but no definitive studies were carried out. It was often thought that the non-linear crystal was simply a non-linear amplifier for fluctuations already present in the undoubled laser. Stability was not the only problem with these devices; crystal damage and other materials problems tended to limit the performance of the devices.
Interest in intracavity doubled lasers was renewed in the 1980s when new non-linear materials and diode-laser pumping techniques became available. One new non-linear material was KTiOPO.sub.4, potassium titanyl phosphate or KTP, a highly non-linear material which was free from many of the mechanical, thermal and optical problems which had plagued earlier materials. Phase-matching in KTP is Type-II, so the simple Brewster plate polarizers used with earlier materials were not adequate.
Using Type-II non-linear crystals, such as KTP, for intra-cavity second harmonic generation (SHG) introduces a variety of polarization related problems. Placing a birefringent crystal in an unpolarized laser cavity often produces undesirable effects because the crystal axis will define two orthogonal polarizations that will, in general, differ in both their optical path lengths and losses. The path length difference leads to two weakly coupled sets of resonant cavity frequencies which often give rise to erratic mode-hopping behavior and output noise. Furthermore, any differences in the relative losses for each polarization tend to result in a laser output which is polarized along one axis of the crystal. Since radiation polarized along two crystal axes is required for Type-II doubling, output radiation polarized along only one axis would prevent or at least degrade the efficiency of the SHG process. Retardation plates have been used to control the polarizations inside the cavity. The issue of noise was not addressed. Typical examples are found in the following U.S. Pat. Nos.: 4,413,342 to Cohen, et. al.; 4,127,827 and 3,975,693 to Barry et al.; 4,617,666; 4,637,026; 4,048,515; and 4,618,957 to Liu.
Baer appears to have been the first to have built a diode-laser pumped Nd:YAG laser which was intracavity doubled with KTP. See for example, U.S. Pat. Nos.: 4,653,056; 4,656,635; 4,701,929; 4,756,003; and 4,872,177. An early cavity used by Baer consisted of an end-pumped Nd:YAG rod, a KTP crystal and a curved reflector, and had no polarization controlling elements. Baer reported the following results: (1) when the laser was operated without an intracavity etalon, it exhibited optical noise having a frequency in the range from about 10 kilohertz to multiples of 100 kilohertz; (2) when an etalon was added to reduce the number of oscillating modes to two, well-defined oscillations in optical power were observed; and (3) when the laser was forced to run in a single mode with an etalon, the output power was stable, but the laser produced very little green output. Baer interpreted his results in terms of a rate equation model which included both sum generation and cross saturation effects. Baer believed that the laser amplitude fluctuations occurred because the system has two non-linear feedback mechanisms operating on two different timescales. He concluded that the oscillations were a fundamental barrier to successful multimode operation of intracavity doubled lasers.
Later designs by Baer added a Brewster plate polarizer oriented at 45.degree. from the axis of the KTP to provide equal power in the two crystal polarizations. This design suffered from the fact that, in general, a Brewster plate and a birefringent crystal cannot be combined in a low-loss optical cavity. The linear polarization passed by the Brewster plate will be transformed by the KTP into an elliptical polarization that will experience a significant loss upon passing through the Brewster plate. We have found that only in the special case (not described or discussed by Baer) when the KTP functions as a half-integral waveplate, will the cavity losses be low. Because KTP is strongly birefringent, having temperature-dependent refractive indices, a typical few millimeter long, doubling crystal of KTP will act as a temperature-variable, multiple-order retardation plate. In general, we have found that for low-loss eigenmodes to exist in a laser cavity containing a Brewster plate and a birefringent element, the birefringent element must be a full-or half-wave plate. Thus, success in producing a low loss optical cavity at a given wavelength is critically dependent upon rigid control of the crystal length and the cavity temperature. A sensitive inter-relationship exists between crystal length, cavity temperature and polarization losses.
Other have also attempted to make a solid-state laser which uses non-linear crystal or lasant material to produce green light from infrared light using the principles of second harmonic generation. The following U.S. patents are illustrative of the many practitioners who have attempted to make a practical apparatus: U.S. Pat. Nos. 3,624,549 to Geusic et al.; 3,750,670 to Palanos et al.; 3,619,637 to Godo et al.; and 4,856,006 to Yano et al.
More recently, Anthon et al. disclosed an intracavity frequency doubled laser (U.S. Pat. No. 4,933,947 and assigned to AMOCO Corporation) having improved amplitude stability. This was achieved by substantially eliminating spatial hole burning in the lasant material and by maintaining the optical cavity of the laser at a temperature which results in substantially noise-free generation of optical radiation.
Despite what appears to be a fairly complete, general understanding of the theory of the frequency doubling process, a reliable, solid-state, diode-laser-pumped, frequency-doubled laser has yet to find complete acceptance in the market place. Heretofore such lasers have been plagued with a variety of problems. These problems have included: a variation in power output during start-up; output powers which vary significantly with changes in temperature and over time; non-repeatable output power with a variation in cavity temperature; multiple (e.g., two or three) spectral modes running simultaneously; differing polarizations in the spectral modes without any consistent relationship between them; an infrared (IR) polarization which was not defined; spectral modes and output powers which change when the laser is tapped or slightly vibrated; and laser operation (i.e., output power and spectral modes) which seem to be unduly sensitive to normally occurring changes in the characteristics of the pumping diode-laser.
There are also problems in the operation of non-frequency doubled, diode-pumped, solid-state lasers. Due to the smallness of the optical wavelengths, a simple laser cavity can support oscillation at many different wavelengths. A laser cavity resonator may oscillate simultaneously at several wavelengths, and in several "temporal modes", or oscillation may alternate between one or more of the modes competing for the gain of the laser. The spectral content of such a laser is irregular; the bandwidth may be large but the intensity is principally divided among the several and sometimes thousands of modes of oscillations. The output pulse intensity from such a multimode, broadband laser is typically characterized by a highly modulated, rapidly fluctuating shape. For many applications, an output at a single wavelength/frequency and mode is desirable. Much research has been voted to design lasers whose output is exactly single wavelength. Those lasers that come closest to a "single wavelength" or "single frequency" oscillate in substantially a single temporal and spatial mode and have a very narrow bandwidth around that single wavelength.
Clearly a reliable and dependable, single-frequency laser would be welcomed by the photonics industry. More importantly, if a solid-state, diode-laser source of infrared optical pumping radiation is used, a miniature source of stable laser light can be obtained.